The rotator cuff is an important group of muscles and tendons that connect the upper arm bone (humerus) to the shoulder blade (scapula). It provides stability to the shoulder joint and allows it to move and rotate. Rotator cuff tendonitis, commonly referred to as shoulder tendonitis, is a condition in which any of these tendons-thick bands of tissue that connect muscles to bones-becomes inflamed or irritated.
The most common symptom of shoulder tendonitis is pain and possibly swelling at the tip of the shoulder and upper, outer part of the arm. This pain often gets worse if you reach, lift, pull, push or lie on your side; this can make it difficult to sleep comfortably if you roll or sleep on your shoulder. There may also be a "clicking" sensation when you reach your arm above your head. Some patients also experience stiffness and loss of mobility and/or strength.